Cyclosiloxanes are often used as starting monomers to form polysiloxanes. Generally polysiloxanes may be formed by the ring opening polymerization of cyclosiloxanes and a suitable end-capping unit. The reaction is carried out in the presence of a catalyst.
Various catalysts are known for the polymerization of cyclosiloxanes. Examples are alkali metal hydroxides, alkali metal alkoxides or complexes of alkali metal hydroxides and an alcohol, alkali metal silanolates, and phosphonitrile halides (sometimes referred to as acidic phosphazenes). Such polymerizations may be carried out in bulk, in solvents (such as non-polar or polar organic solvents) or in emulsion. Phosphazene bases and carbenes have been described as suitable catalyst for the ring opening polymerization of siloxanes. An endblocking agent may be used to regulate the molecular weight of the polymer and/or to add functionality. Polymerization may be terminated by using a neutralizing agent which reacts with the catalyst to render it non-active. In most cases, catalyst residues remain in the polymer product and are desirably removed, such as by filtration.
Solid type catalysts may be used as a catalyst to synthesize polydimethylsiloxane (PDMS) fluids and PDMS functional fluids by ring opening polymerization. Solid type catalysts exhibit high catalytic activity, but generate solid waste. The solid waste is typically incinerated. Additionally, the solid waste contains a significant fraction of the product, which decreases the overall yield and may increase production costs.